Steph- wouldn't it be reasonable to not run at all for a week or 2, shake off this lurgy, and come back once you're properly recovered. I thing trying to get running again this weekend sounds too soon.
I read somewhere that everyone shuod take a month or so off running every year- not that I listen to my own advice, but it is worth a thought.
The main risk of pushing yourself hard in the first few weeks is that your heart and lungs improve quicker thatn the mucles/ joints/ bones in your legs, - it is easy to give yourself a leg injury by over- doing it, and at 6 weeks in, you're crocked. By all means keep going with the C25k, but don't strive for a PB every time you go out- slow and steady wins the race!
Why not do a park run every month or so, rather than every week? - otherwise you'll just have a nother few days of being knackered after the event, and you'll not be able to keep doing that regularly without damaging your self.
You can always offer to volunteer at the parkruns too- beingpart of the park run "family" will keep you enthused, as well as doing the runs.